As is known in the art, Atomic Layer Deposition (ALD) technique is used in the deposition of thin films of metals and dielectric in which there is precise, atomic level, control of the film thickness and conformality. This feature is due to the reaction limited chemistry of the deposition technique. The ALD method has been applied to the formation of dielectric capacitors in DRAM manufacturing and recently in the form of barrier metal nitrides, TiN and TaN, in the manufacture of Si integrated circuits (ICs) with hundreds of millions of transistors. Not every metal or dielectric can be deposited using ALD method; however. Among the metals which can be routinely produced by ALD, TiN and Ni are of particular interest to the formation of the gate region of compound semiconductors, and in particular, III-V semiconductors.
Presently, the gate of most mature III-V FETs, GaAs and InP, are fabricated using a Tee-gate structure in a double recessed device. As GaN and future semiconductor technologies, e.g., diamond mature to the manufacturing level, a double recessed structure will be employed in their fabrication. A schematic diagram of a double recessed Tee-gate structure is shown in FIG. 1 having N+/N GaAs Contact Layer 28, AlGaAs Top Confinement Layer 26, Top Si Pulse Doping (˜5 Å) layer 24, InGaAs Pseudo-morphic Channel layer 22, Bottom Si Pulse Doping (˜5 Å) layer 20, AlGaAs Bottom Confinement Layer 18, AlGaAs/GaAs Supper Lattice Buffer Layers 16, GaAs Buffer Layer 14, and semi-insulating GaAs substrate 12. The FET includes source and drain electrodes 30 in ohmic contact with the N+/N GaAs Contact Layer 28. The structure includes a first recess 34 formed in the N+/N GaAs Contact Layer 28. Following the formation of the first recess 34 in the semiconductor structure, as shown in FIG. 1, a thin layer or film 37 of dielectric, such as SiN is deposited over the surface of such semiconductors structure. A Tee-gate 36 is formed then through a second recess 38, such Tee-gate being in Schottky contact with the AlGaAs Top Confinement Layer 26. It is noted in FIG. 1A that there are air gaps in recess 38, such air gaps being between the semiconductor and the Tee gate 36.
The main electrical disadvantage of the Tee-gate is the significant increase in the parasitic gate to drain and gate to source capacitances upon passivation of the FETs by common dielectrics such as SiN. This problem gets progressively worse as more layers of passivation are needed for the interlayer dielectric and environmental protection of the chip. Furthermore, there are numerous processing difficulties associated with the Tee-gate fabrication in a double recessed structure as follows.                1. When a direct E-beam write method is used to form the Tee-gate, two separate E-beam resist, such as PMMA and PMAA have to be spun and written with different dose and shapes. This reduces the throughput to less than 4 wafers per hour. Furthermore, the control of the gate length and Tee-top dimensions are hard to achieve. This issue has been discussed in a paper by K. Tabatabaie-Alavi, D. M. Shaw, and P. J. Duval, “Evolution of T-Shaped Gate Lithography for Compound Semiconductors Field Effect Transistors,” in IEEE Transaction on Semiconductor Manufacturing, August 2003, pp. 365-369.        2. Even when the Tee-top and stem are written separately in a hybrid Tee-gate approach, the alignment between the two resist levels and intermixing between the two resist layers have to be tightly controlled.        3. Due to the nature of the evaporated Ti/Pt/Au (GaAs, InP) or Ni/Pt/Au (GaN, AlGaN), severe crevices are formed underneath the Tee-gates which impede the coverage of the SiN passivation layer.        4. When a dielectric is used for the transfer of the stem pattern in a hybrid gate approach (see D. Fanning, L. Withowski, J. Stidham, H.-Q. Tseng, M. Muir, and P. Saunier, “Dielectrically defined optical Tee-gate for high power GaAs pHEMT,” in Proc. GaAs MANTECH, 2002, pp. 83-86), the etching of the dielectric and the subsequent semiconductor (gate recess) become extremely critical because any excessive amount of etch undercut or damage could severely impact device performance and long term stability.        